The present invention relates generally to an end-face reflecting-type surface wave resonator using an SH-type surface wave, such as the BGS wave, the Love wave, or the like. More particularly, the invention relates to an end-face reflecting-type surface wave resonator in which a plurality of resonance units are formed into a single device.
At the video intermediate frequency stage of a television receiver or a video cassette recorder, in order to prevent interference with adjacent channels, it is necessary to sufficiently attenuate signals at the adjacent-channel video signal frequency fap (31.9 MHz according to the European PAL system) and the adjacent-channel sound signal frequency fas (40.4 MHz according to the European PAL system). FIG. 1 is a characteristic diagram of a typical filter indicating attenuation versus frequency at the video intermediate frequency stage. FIG. 1 reveals that a great level of attenuation is conventionally provided at the adjacent-channel video signal frequency fap and the adjacent-channel sound signal frequency fas.
In order to guarantee a great level of attenuation at both the adjacent-channel video signal frequency fap and the adjacent-channel sound signal frequency fas, two trap devices are conventionally employed, such as a trap device having a great level of attenuation at the frequency fap and another trap device having a great level of attenuation at the frequency fas. Each trap device is formed of an LC resonance circuit, a piezo-resonator, or the like.
As a piezo-resonator for the above-described use, attention is being focussed on a piezo-resonator utilizing an SH-type surface wave, such as the BGS wave. FIG. 2 illustrates an example of an end-face reflecting-type surface wave resonator using the BGS wave.
The end-face reflecting-type surface wave resonator generally designated by 1 has a piezoelectric substrate 2 formed in a quadrilateral planar shape. The piezoelectric substrate 2 is formed of a piezoelectric material, such as lead zircotitanate piezoelectric ceramics, LiNbO.sub.3 piezoelectric single crystal, LiTaO.sub.3 piezoelectric single crystal, etc; the substrate 2 formed of piezoelectric ceramics having been polarized in the direction indicated by the arrow P shown in FIG. 2. Disposed on the top surface 2a of the substrate 2 are a pair of comb-like electrodes 3 and 4 which form an interdigital transducer (hereinafter simply referred to as "the IDT"). The comb-like electrodes 3 and 4 have a plurality of electrode fingers 3a through 3c and 4a through 4c, respectively.
In the surface wave resonator 1 constructed as described above, an AC voltage is applied to the resonator 1 from the comb-like electrodes 3 and 4 to excite the BGS wave which then propagates in the direction indicated by the arrow X and is further reflected between the end faces 2b and 2c. In this resonator 1, the frequency determined by the IDT is matched to the frequency determined by the distance between the end faces, thereby obtaining effective resonance characteristics.
However, the end-face reflecting-type surface wave resonator described above, as well as conventional LC resonance circuits and other types of piezo-resonators, have only a single resonance. Accordingly, two surface wave resonators are required and must be connected to each other in order to achieve trap characteristics at both the adjacent-channel video signal frequency fap and the adjacent-channel sound signal frequency fas.
On the other hand, a single surface-acoustic wave resonator using the Rayleigh wave exhibiting two types of resonance characteristics is disclosed (for example, in Kokusai Electric Co., Ltd., Technical Report No. 16, pages 1-7, 1992). More specifically, in a surface wave resonator filter using the Rayleigh wave, a double mode resonator using a 0th-order longitudinal mode (dominant mode) and a second-order longitudinal mode is known, exhibiting two types of resonance characteristics.
However, in this type of resonator, at least two sets of IDTs and reflectors are required to obtain the two types of resonance characteristics. Also, the resonance characteristics of the double mode resonator are determined by the reflection coefficient versus the frequency characteristics of the reflectors. However, because the resonator has a narrow frequency range having a large reflection coefficient, there is only a very small difference, such as approximately 1 MHz, between two resonance points in a region in which good resonance characteristics are obtained. This makes it difficult to provide the two required trap resonators merely by use of such a double mode resonator, since the two trap resonators have very different frequencies at which the attenuation characteristics shown in FIG. 1 are exhibited.
One of the measures that have been considered to overcome the above-described drawback and to obtain two types of resonance characteristics may be to form two IDTs on a piezoelectric substrate so as to constitute two resonance units, in an end-face reflecting-type surface wave resonator discussed above. However, in the above type of resonator using an SH-type surface wave, such as the BGS wave, the relationship between the wavelength .lambda. of the IDT and the distance L between the oppositely-facing two end faces of the piezoelectric substrate between which the surface wave is reflected can be expressed by L=(.lambda./2).times.n (wherein n is an integer), as schematically illustrated in FIG. 3. A great difference between the adjacent-channel video signal frequency fap and the adjacent-channel sound signal frequency fas causes a significant difference between the respective wavelengths .lambda.. For example, when it is assumed that the speed of sound in the PZT substrate is 2400 m/s, according to the PAL system, the wavelength .lambda.ap for the adjacent-channel video signal frequency fap is 75.2 .mu.m, while the wavelength .lambda.as for the adjacent-channel sound signal frequency fas is 59.4 .mu.m. Accordingly, the distance L between the above-described oppositely-facing end faces of one resonance unit having a resonance point at the adjacent-channel video signal frequency fap differs from that of the other resonance unit having a resonance point at the adjacent-channel sound signal frequency fas. Thus, it is very difficult to form two resonance units having different resonant frequencies on the same substrate of an end-face reflecting-type surface wave resonator using the SH-type surface wave.